1. Field of The Invention
The present invention relates to a novel blocked polyisocyanate and use thereof. More particularly, the present invention is concerned with a novel blocked polyisocyanate obtained by blocking the terminal isocyanate groups of a non-blocked polyisocyanate with a thermally dissociative blocking agent, wherein the non-blocked polyisocyanate is obtained by reacting at least one diisocyanate selected from the group consisting of an aliphatic diisocyanate and an alicyclic diisocyanate with a polyhydroxy compound having an average number of hydroxyl groups as many as 4.5 to 10, and removing substantially all unreacted diisocyanate monomers; and is also concerned with use of the blocked polyisocyanate.
The use of the blocked polyisocyanate of the present invention includes, for example, a coating composition comprising a polyol as a main agent and the blocked polyisocyanate as a curing agent; an adhesive property-imparting composition comprising the blocked polyisocyanate, and a plasticizer and/or a solvent which are/is for the blocked polyisocyanate; and a plastisol composition comprising a plastisol (which is a dispersion of a vinyl chloride polymer in a plasticizer) and the adhesive property-imparting composition. The coating composition comprising the blocked polyisocyanate of the present invention as a curing agent has not only excellent curability even under low temperature conditions, but also is capable of forming a coating which has excellent impact resistance. Therefore, the coating composition of the present invention can be advantageously used for coating a metallic substrate (such as a steel plate or a surface-treated steel plate) and a plastic substrate, by the technique of roll coating, curtain flow coating or the like. Especially, the coating composition of the present invention is useful as materials for undercoating, top coating and intercoating various substrates; a coating material for a precoated metal including a rust-resistant steel plate; and a coating material for imparting fine appearance, weatherability, acid resistance, rust resistance, mechanical strength (such as chipping resistance and impact resistance) and the like to automobiles. The blocked polyisocyanate of the present invention has excellent miscibility with a plasticizer for the blocked polyisocyanate, so that, an adhesive property-imparting composition comprising the blocked polyisocyanate and a plasticizer and/or a solvent, can be combined with a plastisol to prepare a plastisol composition having excellent adhesive properties under low temperature conditions. This plastisol composition can be advantageously used, for example, as sealants, materials for forming an undercoating and a chip-resistant coating, and a material for the preparation of a PVC-coated steel plate in the fields of automobiles, wherein the plastisol composition is applied to by the technique of spraying, brushing or the like.
2. Prior Art
Energy-saving and the prevention of air pollution are important tasks in the field of coating. In this field, it has been desired to diversify and improve the functions of coating compositions and plastisols.
A blocked polyisocyanate, which is obtained by blocking the terminal isocyanate groups of a non-blocked polyisocyanate derived from an aliphatic and/or an alicyclic diisocyanate with a blocking agent, is stable at ambient temperature. However, upon heating the blocked polyisocyanate, the blocking groups dissociate from the terminal isocyanate groups of the blocked polyisocyanate, and the resultant deblocked polyisocyanate can serve as a curing agent for a polyol. In view of the above-mentioned properties, the blocked polyisocyanate has been expected to be a promising curing agent being capable of changing the type of a polyurethane coating composition from a two-pack type to a one-pack type, wherein the polyurethane coating composition comprises a polyol as a main agent and the blocked polyisocyanate as a curing agent, and is capable of providing a coating having excellent properties, such as chemical resistance, flexibility and weatherability. However, with respect to the blocked polyisocyanate, it has been still desired to improve the properties thereof, so as to not only solve the above-mentioned tasks, but also diversify and improve the function thereof.
With respect to the blocked polyisocyanate, which has conventionally been used as a curing agent for a one-pack polyurethane coating composition, it is necessary to heat the coating composition to high temperatures for releasing a blocking agent from the terminal isocyanate groups of the blocked polyisocyanate so as for the polyisocyanate to function as a curing agent. For example, when a coating is formed by baking from a one-pack polyurethane coating composition comprising a polyol as a main agent and a blocked polyisocyanate as a curing agent, the baking must be conducted at a temperature as high as 150 to 200.degree. C. Such a high baking temperature is disadvantageous in that not only does it take a high cost to conduct the baking, but also a problem of air pollution is caused due to the high temperature baking. Such high baking temperatures cause difficult problems. For example, when a substrate having a low heat distortion temperature (such as a plastic) is coated with the above-mentioned coating composition, a special jig device needs to be generally used in the baking procedure for preventing the heat distortion of the substrate; however, in some cases, even such a special jig device is not useful in the baking procedure.
Thus, the use of the conventional blocked polyisocyanate has been limited, and it has earnestly been desired to develop a novel blocked polyisocyanate which can function as a curing agent for a polyol even under relatively low temperature conditions such that the above-mentioned problems are not caused, that is, a temperature as low as 110 to 130.degree. C. (which is frequently desirable from a practical point of view).
In these situations, several proposals have been made for lowering the baking temperature necessary for curing the above-mentioned coating composition, that is, for imparting the coating composition with a curability under relatively low temperature conditions (hereinafter, frequently referred to simply as "low temperature curability"). For example, for imparting a low temperature curability to a coating composition there are some proposals of improving a curing accelerator which is generally contained in the coating composition. As an improved curing accelerator, for example, Examined Japanese Patent Application Publication No. 44-18877, Unexamined Japanese Patent Application Laid-Open Specification No. 53-138434, Unexamined Japanese Patent Application Laid-Open Specification No. 56-84714 (corresponding to G.B. Patent No. 2065151) and Unexamined Japanese Patent Application Laid-Open Specification No. 57-8217 (corresponding to G.B. Patent No. 2065151) disclose an organotin compound; Unexamined Japanese Patent Application Laid-Open Specification No. 62-199609 discloses a lead compound, an inorganic zinc compound or a mixture thereof; and Unexamined Japanese Patent Application Laid-Open Specification No. 2-199112 discloses a mixture of an organotin compound and at least one compound selected from the group consisting of a cobalt compound, a nickel compound and a zinc compound. However, a desired low temperature curability of the coating composition cannot be achieved by improving only a curing accelerator.
As an alternative measure for imparting a low temperature curability to a urethane coating composition, there have been some proposals of using an improved blocking agent for a non-blocked polyisocyanate. As an improved blocking agent, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 60-149572 discloses an alkyl acetoacetate, Unexamined Japanese Patent Application Laid-Open Specification No. 6-287269 discloses a mixture of an oxime compound and an active methylene compound; Unexamined Japanese Patent Application Laid-Open Specification No. 7-304843 (corresponding to Canadian Patent No. 2148987) discloses a mixture of a triazol or a derivative thereof with a pirazole or a derivative thereof; and Unexamined Japanese Patent Application Laid-Open Specification No. 7-216051 (corresponding to Canadian Patent No. 2139365) discloses a pirazoline or a derivative thereof. However, these proposals concerning a blocking agent are also unsatisfactory for achieving a low temperature curability of the coating composition.
In addition to the above-mentioned methods in which it is attempted to lower a curing temperature for a urethane coating composition by using an improved curing accelerator or an improved blocking agent, Unexamined Japanese Patent Application Laid-Open Specification No. 6-293878 (corresponding to International Patent Application Publication No. WO 94/18254) discloses a method for lowering a curing temperature for a urethane coating composition by using as a curing agent a multifunctional polyisocyanate, i.e., a polyisocyanate having an isocyanurate skeleton, which has a number of terminal isocyanate groups. However, the curing agent used in this method is still unsatisfactory for avoiding the above-mentioned problems of a high cost for the baking for forming a coating, an air pollution caused by the baking and the like, with respect to a polyester polyol, an epoxy polyol and the like which are used as main agents in urethane coating compositions.
In the field of coating, as mentioned above, it has also been desired to diversify the function of a coating composition. For example, in cold areas, especially North America and north Europe, rock salt and gravel are scattered on a road for the purpose of preventing the road from freezing. However, the scattered rock salt and gravel are caused to leap and fly by the turning wheels of cars, and such rock salt and gravel frequently hit the cars, leading to various problems, such as occurrence of scratches on car bodies. During wintertime in the cold areas, the temperature frequently drops to -20.degree. C. or lower. Under such low temperature conditions, coatings formed from conventional urethane coating compositions do not absorb impact caused by rock salt and gravel. In these situations, it has been strongly desired to develop a onepack type coating composition which has not only a curability even at a temperature as low as 110 to 130.degree. C. which is frequently desirable from a practical point of view, but also can provide a high impact resistance coating capable of protecting a car body from the impact caused by rock salt and the like. However, no one-pack type urethane coating composition having excellent impact resistance has been reported.
In the field of coating compositions and sealants, a so-called plastisol which is a dispersion of a vinyl chloride polymer in a plasticizer is used. However, when a plastisol is coated on a metallic substrate, the plastisol does not exhibit satisfactory adhesion to the metallic substrate. Therefore, an improvement in adhesive property of the plastisol has been desired. Conventionally, for improving adhesive property of a plastisol, an adhesive property-imparting composition is added to the plastisol, which comprises a blocked polyisocyanate and/or a polyamide amine. With respect to the improvement in adhesive property by the use of the adhesive property-imparting composition, various proposals have been made. As such an adhesive property-imparting composition, for example, Examined Japanese Patent Application Publication No. 59-52901, discloses a polyamide amine and/or a blocked urethane prepolymer which is obtained by blocking the terminal isocyanate groups of a urethane type isocyanate prepolymer produced from a polyol and a diisocyanate with a blocking agent; Examined Japanese Patent Application Publication No. 62-41278 discloses a blocked polyisocyanate obtained by blocking with a long-chain alkyl phenol a polymer of the diisocyanate, having an average molecular weight of from 1,000 to 10,000; Unexamined Japanese Patent Application Laid-Open Specification No. 1-170633 discloses a polyamine and/or an urethane type prepolymer of a blocked polyisocyanate having an isocyanurate skeleton; and Examined Japanese Patent Application Publication No. 3-29257 (corresponding to U.S. Pat. No. 5,043,379) discloses a blocked polymer of an aromatic diisocyanate obtained using at least two types of alkylphenol blocking agents. The polyamide amines and polyamines proposed in the above documents have excellent adhesive property-imparting ability. However, they have poor water resistance, yellowing resistance and the like. In addition, the above-mentioned blocked polyisocyanates have problems in that, although they have excellent water resistance, they are incompatible with a plasticizer for a vinyl chloride polymer plastisol and have unsatisfactory adhesive properties under low temperature conditions, and thus, the use of such blocked polyisocyanates have been limited.
In British Patent No. 994890, a polyisocyanate obtained from a polyether polyol and a diisocyanate is disclosed. However, this document has no description or suggestion about a blocked polyisocyanate obtained by blocking the polyisocyanate, and a one-pack type coating composition containing such a blocked polyisocyanate. Needless to say, this document has no description about the properties of the coating composition, such as a low temperature curability.
Unexamined Japanese Patent Application Laid-Open Specification No. 7-304724 (corresponding to Canadian Patent No. 2148783) discloses a method for producing a polyisocyanate having an allophanate bond, in which a urethane bond-containing polyisocyanate (obtained by reacting a polyhydroxy compound with an aliphatic diisocyanate and/or an alicyclic diisocyanate) is reacted with an aliphatic diisocyanate and/or an alicyclic diisocyanate in the presence of a tin compound. Each of the polyhydroxy compounds used in Examples of the above document has an average number of hydroxyl groups of only 1 or 2, and there is no description about a polyhydroxy compound having a large average number of hydroxyl groups, such as an average number in the range of from 4.5 to 10. Further, there is no description about the use and effects of a specific blocked polyisocyanate obtained using as a modifier a polyhydroxy compound having such a large average number of hydroxyl groups.
Unexamined Japanese Patent Application Laid-Open Specification No. 2-132116 (corresponding to U.S. Pat. No. 4,762,752) discloses a polyisocyanate having a relatively large average number of isocyanate groups. The average number of isocyanate groups per molecule of the polyisocyanate is from 3 to 6. However, the polyisocyanates having an average number of isocyanate groups of 4 or more disclosed in the above-mentioned document have aromatic isocyanate groups and hence have poor durability.